The present invention relates to laminated multifocal lenses and, more particularly, to composite plastic lenses with an outer veneer lens carrying a bifocal element and an inner lens which is the prescription base.
The formation of bifocal lenses has been a generally difficult and expensive undertaking because of the variety of different and conflicting requirements which must be mutually reconciled in forming each lens. The expense involved has been relatively high because these lenses are commonly formed from glass and CR39 (allyl diglycol), a cast molded plastic, the grinding of which is very expensive and difficult. The conflicting requirements necessary for each lens in view of the great number of different combinations of compound bifocal prescriptions as well as cosmetic requirements have also contributed to the high cost of bifocal lenses.
In an effort to reduce the weight of bifocal lenses as well as manufacture the same more rapidly and inexpensively, multifocal lenses have been molded from thermoplastic resins such as polycarbonate resin. In order to be commercially acceptable, however, lenses must be free of blemishes and distortions and have edge proportions free from starring that can result from non-uniform setting within the mold cavity. These optical properties of molded lenses are very difficult to control. In particular, in order to ensure adequate optical and other physical properties of such a molded lens, a number of parameters, including the quantity and temperature of the resin, mold temperature, clamp pressure, speed and pressure with which the resin is injected, pressure that the molding members apply to the resin entrapped in the fill cavity and the duration of the various molding steps must be controlled.
In addition, it is crucial that polycarbonate lenses, for example, be produced having a minimum of stress. One approach that minimizes stress is uniform application of molding pressure in the direction of the thickness of the lens blank throughout the extent of the major surfaces of the blank, at least during the period of time when the resin is setting. In order to obtain a uniform application of molding pressure, the charge of molten thermoplastic material must fill the cavity completely so that the opposite molds will engage throughout their entire extent at both major surfaces of the molten charge. This engagement also ensures that the lenses being formed are not stressed along the edge portions during molding. Accordingly, starring is minimized. The minimization of starring is important since when an optometrist edges or machines a lens blank to conform to the shape of an eyeglass frame, the presence of starring and the resultant edge weakness, may lead to breakage or distortion of the lens.
An additional consideration when bifocal lens are being injection molded is that it is imperative that extremely accurate positioning of the bifocal segment in relation to the spherical base curve be maintained. During the injection process both the bifocal segment and spherical base curve are subjected to pressures of varying intensity which can cause deflection of the bifocal segment relative to the base curve. When this occurs, the lens can flash or an uneven transition point to or from the bifocal segment can be produced which will render the lens useless. Furthermore, since these pressures can vary due to several parameters, continuous adjustment of the bifocal segment position is required. The resultant down time of injection molding machines while the mold is disassembled in order to readjust the bifocal segment becomes a significant detriment to machine efficiency.
It would be desirable to provide laminated bifocal lenses that are lightweight, inexpensive, and can be readily formed in accordance with a given prescription from a relatively small number of component lenses maintained in stock.
It would also be desirable to form laminated bifocal lenses from thermoplastic components so that they can be inexpensively and rapidly produced.
It would also be desirable to be able to form the component portions of such a laminated bifocal lens so that they are substantially free from stress, as well as blemishes and distortions, and hence have optimum optical characteristics.
It would further be desirable to provide an adjustment mechanism for the position of the bifocal segment relative to the spherical base curve that is externally accessible so that the bifocal segment can be readjusted without necessitating disassembly of the molding station. Such an adjustment mechanism would also allow optimum surface transition to be provided by allowing the user to compensate for the deflection of the bifocal segment in relation to the base curve.
Finally, it would be advantageous to provide a molding apparatus capable of molding bifocal segments of varying powers on a given base curve by merely interchanging the bifocal segment and readjusting the segment position.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a laminated bifocal optical lens which overcomes the aforementioned deficiencies of the prior art.
According to the present invention, an ophthamalic lens is formed by permanently bonding two thermoplastic components together, such as a meniscus carrying a bifocal capability on the front surface bonded to a rear meniscus carrying the optical prescription on its rear surface. The arced inner surface of the bifocal meniscus has the same curvature as the front surface of the rear meniscus so that a particular bifocal element can be chosen and mounted to a particular base prescription in accordance with a given patient's prescription.
Further, each of the lenses forming the bifocal laminated lens of the present invention is formed by a molding apparatus and process which enable the uniform application of pressure to the lens blank and, hence, is substantially stress-free.
Other objects, features, and characteristics of the present invention, as well as the methods of operation and functions of the related elements of the structure, and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures.